1. Field of the Invention
The present invention relates to methods and apparatus for extracting carbon dioxide and hydrogen from seawater and to processes for hydrocarbon production including the carbon dioxide extraction method.
2. Description of the Related Technology
It is desirable to be able to produce jet fuel at sea to support aircraft carrier flight operations. In-theater, synthetic fuel production would offer significant logistical and operational advantages by reducing dependence on increasingly expensive fossil fuels and by reducing the vulnerabilities resulting from unprotected fuel delivery at sea. A ship's ability to produce a significant fraction of the battle group's fuel for operations would increase the operational flexibility and time on station by reducing the mean time between refueling.
Technologies currently exist to synthesize hydrocarbon fuel on land, given sufficient primary energy resources such as coal. Davis, B. H. Topics in Catalysis 2005, 32, 143-168. However, these technologies are not CO2 neutral, and they are not practical for sea-based operation. Extracting carbon dioxide from seawater is part of a larger project to create liquid hydrocarbon fuel at sea. Hardy, D. H., et al., “Extraction of Carbon Dioxide From Seawater by Ion Exchange Resin Part I: Using a Strong Acid Cation Exchange Resin,” Memorandum Report 6180-07-9044; Naval Research Laboratory: Washington D.C., Apr. 20, 2007; Willauer, H. D., et al., “Recovery of CO2 from Aqueous Bicarbonate Using a Gas Permeable Membrane,” Energy & Fuels, 2009, 23, 1770-1774; Willauer, H. D., et al., “Recovery of [CO2]T from Aqueous Bicarbonate Using a Gas Permeable Membrane,” NRL Memorandum Report, 6180-08-9129, 25 Jun. 2008; Willauer, H. D., et al., “Extraction of CO2 From Seawater By Ion Exchange Resin Part II: Using a Strong Base Anion Exchange Resins,” NRL Memorandum Report, 6180-09-9211, 29 Sep. 2009; Dorner, R. W., et al., “Influence of Gas Feed Composition and Pressure on the Catalytic Conversion of CO2 Using a Traditional Cobalt-Based Fischer-Tropsch Catalyst,” Energy & Fuels, 2009, 23, 4190-4195; Dorner, R. W., et al., “Effects of Loading and Doping on Iron-based CO2 Hydrogenation Catalyst,” NRL Memorandum Report, 6180-09-9200, 24 Aug. 2009; Dorner, R. W., et al., “Mn doped Iron-based CO2 Hydrogenation Catalysts: Detection of KAlH4 as part of the catalyst's active phase,” Applied Catalysis A, Jan. 2010, Vol. 373, Issues 1-2, pp. 112-121; and Willauer, H. D., et al., “The Effects of Pressure on the Recovery of CO2 by Phase Transition from a Seawater System by Means of Multi-layer Gas Permeable Membranes,” J. Phys. Chem. A, 2010, 114, 4003-4008. CO2 as a carbon feedstock could be catalytically reacted with hydrogen to form diesel and/or jet fuel. The hydrogen could be produced through commercial off the shelf conventional electrolysis equipment, and the electrical energy for this process would be derived through nuclear power or Ocean Thermal Energy Conversion (OTEC). Mohanasundaram, S. Renewable Power Generation-Utilising Thermal Energy From Oceans. Enviro. Sci. & Eng. 2007, 4, 35. Avery, W. H.; Wu, C. Renewable Energy From The Ocean; Oxford University Press: New York, 1994. This synthetic fuel production process could provide an alternative energy source to fossil fuels.
Practical, efficient, and economical methods of extracting large quantities of CO2 from seawater must be developed before a sea-based synthetic fuel process that combines hydrogen produced by nuclear power or solar OTEC with CO2 to make jet fuel can be envisioned. The ocean's pH is kept relatively constant at approximately 7.8 by a complex carbonate buffer system. 96% of the carbon in the oceans is in the form of HCO3−. At pH of 4.5, 99% of all carbonate species in seawater exist as H2CO3. Thus, in order to convert HCO3− to H2CO3, the pH of seawater must be lowered.
CO2 dissolved in water is in equilibrium with H2CO3 as shown in equation 1 as:CO2+H2O H2CO3  (1)The hydration equilibrium constant is 1.70×10−3. This indicates that H2CO3 is not stable and gaseous CO2 readily dissociates at pH of 4.5, allowing CO2 to be easily removed by degassing once the seawater has been acidified to ensure that the unstable H2CO3 is the predominant carbonate species, as discussed above.
A detailed composition of seawater shows a carbon concentration of 28 ppm (˜100 mg/L as CO2). Assuming that the carbon exists as HCO3−, the HCO3− concentration in seawater will be approximately 142 ppm (0.0023 M), therefore approximately 23 mL of 0.100 M hydrochloric acid is required per liter of seawater:HCl+HCO3−→H2CO3+Cl−  (2)
CO2 exists only in the dissolved gas form when the pH of seawater is decreased to 6 or less Johnson, K. M., King, A. E., Sieburth, J. Coulometric TCO2 Analyses for Marine Studies: An Introduction. Marine Chem. 1985, 16, 61. A strong cation exchange resin material can be used to acidify the seawater to below pH 6. Hardy, D. H. Zagrobelny, M.; Willauer, H. D.; Williams, F. W. Extraction of Carbon Dioxide From Seawater by Ion Exchange Resin Part I: Using a Strong Acid Cation Exchange Resin; NRL Memorandum Report 6180-07-9044; Naval Research Laboratory Washington D.C., Apr. 20, 2007. However, the volume of water per unit weight of resin required to regenerate the resin was much larger than the volume of CO2 recovered, and potentially larger than the volume of fuel produced from the CO2. As a result the approach was deemed impractical for a sea-based application.
Thus as one further avenue to exploit the pH as a means to recover carbon from the sea, an electrochemical acidification cell that is able to decompose water into H+, H−, hydrogen and oxygen gas by means of electrical energy has been developed and tested. The effects of acidification cell configuration, seawater composition, flow rate, and current on seawater pH are discussed. These data are used to determine the feasibility of this approach for a carbon capture process.
The approach described within is considerably different than traditional electrolysis methods that are specifically tailored toward the production of hydrogen and oxygen from seawater and or fresh water. Most modern military submarines generate their breathing oxygen from the electrolysis of fresh water. The difficulty with current seawater electrolysis technology for hydrogen production is the formation of chlorine gas and thus electrodes are modified such that only oxygen is evolved at the anode. Kato, Z., et al., “Energy-Saving Seawater Electrolysis for Hydrogen Production,” J. Solid State Electrochem., 2009, 13, 219-224.